Various labels have been attached to articles so that the articles can be distinguished one from the other. For example, bar code labels are attached to articles of grocery and are scanned at a check-out counter in order to automatically identify the articles and to register the price of the articles as they are purchased.
Bar code labels have also been used in inventory control and monitoring. Accordingly, these bar codes may be scanned in order to track articles as they move into, through, and out of a storage area. It is also known to read the bar codes attached to articles in order to access various computer records regarding the articles.
Bar code labels, however, have several drawbacks. For example, computer stored records that are accessed when a bar code is read do not move with the corresponding article. Therefore, if the article to which the bar code label is attached is remote from the computer, the records concerning that article cannot be immediately accessed if necessary.
Moreover, bar code labels cannot be read remotely. Thus, if it is desired to take an inventory of articles currently in the storage area, personnel must physically scan each label on each article one at a time in order to determine which articles are presently in the storage area. Such scanning requires the physical presence of the personnel at the location of the articles and is extremely time consuming. Additionally, because bar code labels cannot be read remotely, they cannot be used as security devices that can be detected if the articles to which they are attached are improperly removed from a secured area.
Instead of bar coded labels, it is known to attach radio frequency identification (RFID) tags to the articles to be monitored. The RFID tags can be read, as can bar code labels. However, unlike bar code labels, reading RFID tags does not require the physical presence of personnel because the RFID tags can instead be read remotely. Thus, inventory can be taken more quickly because personnel are not required to walk around a storage area or other area in order to read the RFID tags. Moreover, because RFID tags can be read remotely, they can be used as security devices. Thus, if someone attempts to surreptitiously remove an article to which an RFID tag is attached from a secured area, a remote reader can sense the RFID tag and provide an appropriate alarm.
RFID tags can be read one at a time or in groups. When multiple RFID tags in a group are read at the same time, the information transmitted by the multiple tags frequently collide. Accordingly, spread spectrum techniques, such as either direct sequence spread spectrum (DSSS) or frequency hopping, in the communications between the reader and the tags have been suggested in order to reduce the impact of such collisions. It is also known to interrogate a tag using either a direct sequence spread spectrum (DSSS) signal or a frequency hopping signal.
An RFID tag requires a power source in order to permit the transmission of information from the tag to a reader. Traditionally, an RFID tag is powered either locally or remotely. In remote powering of an RFID tag, the RFID tag typically derives its power from the signal transmitted by the reader. A capacitor or other similar storage device stores the power and supplies the stored power to the processing, memory, and transceiver of the RFID tag. One disadvantage of this powering technique is that, if the RFID tag is operated too far away from the reader, the RFID tag cannot derive sufficient energy from the reader's signal to effectively power its components.
Local powering of the RFID tag usually involves using a battery on the RFID tag in order to power the processing, memory, and transceiver. While the use of a local battery overcomes the disadvantage of operating the RFID tag too far away from the reader when the RFID tag is deriving its power from the reader's signal, the use of a local battery has the disadvantage that it requires frequent replacement or re-charging due to the amount of power consumed by the processing, memory, and transceiver of the RFID tag.
It is known to duty cycle a portion of a receiver of a tag in order to conserve battery power. An example of such a receiver is a super-regenerative receiver. The receiver includes an amplification stage, a local oscillator, a quench frequency source, and a detector stage. The tag also includes a microcontroller, an RF transmitter, and an antenna. The tag remains in a low-power quiescent stand-by state until it is activated by a signal from the reader. Following transmission of an activation signal by the reader, the reader sends a request for information. Specifically, the receiver is duty cycled in order to provide quiescent operation with a low current draw. Thus, some elements of the receiver are completely shut down to save tag power while the tag is operating in a quiescent state. Amplifiers of the receiver utilize forward-biased transistor stages whose forward biasing is provided at the duty cycle rate such that power draw is limited due to the duty cycle of the bias. A duty cycle of 1 to 5% is thought to provide sufficient time for reception of the activation signal and to reduce total current draw.
However, such an arrangement has a number of problems. For example, power consuming elements of the tag and even of the receiver may not be shut down during the OFF times of the duty cycle. Accordingly, power is still wasted. Also, if the transmitter of the tag is capable of using several frequencies specified by the reader, the tag described above cannot quickly acquire the desired frequency for its transmitter and must remain on for a sufficient period of time in order to permit acquisition of that frequency.
Moreover, when a tag is turned off and on in order to conserve battery power, or when the tag is infrequently interrogated by a tag reader, or otherwise, it has periods of OFF time during which it is not or cannot be interrogated by a tag reader. Therefore, interrogations by the tag reader can occur only at widely separated interrogation intervals, thereby allowing an article to which the tag is attached to be removed following one interrogation without anyone realizing it until a succeeding interrogation.
The present invention overcomes one or more of these or other problems.